1. Field of the Invention
The present invention generally relates to voltage waveform measuring methods and apparatuses, and more particularly to an electrooptic voltage waveform measuring method and an electrooptic voltage waveform measuring apparatus which are suited for automatically measuring a wiring voltage waveform within a semiconductor large scale integrated circuit (LSI) device using electrooptic effect.
2. Description of the Related Art
When designing and producing semiconductor devices such as semiconductor LSI devices, it is essential that the voltage waveforms applied to the wirings within the semiconductor devices are accurately determined. The width of the wiring layer is becoming narrower with the improved integration density of the semiconductor devices, and it is becoming more and more difficult to cope with the conventional method of measuring the voltage waveform by contacting a probe on a fine wiring layer by use of an optical microscope.
Hence, proposals have been made to use a surface form measuring apparatus which can detect fine structures, in place of the optical microscope, and to measure the voltage waveform by contacting a fine probe of a voltage waveform detection apparatus to the wiring layer. An atomic force microscope (AFM) and a scanning tunnel-electron microscope (STM) are examples of the surface form measuring apparatus which can detect the fine structures.
When using a combination of the above described surface form measuring apparatus and the voltage waveform detection apparatus to measure the voltage waveform applied to the fine wiring layer, it is regarded more promising to use the AFM as the surface form measuring apparatus because the AFM can measure the form even if an object surface to be measured is an insulator.
On the other hand, it has become difficult to carry out an accurate measurement by an electrical measuring technique, and proposals have been made to enable measurement of a high-speed signal in a fine measuring region using the electrooptic effect of electrooptic crystals. For example, such a technique is proposed in Valdmanis et al., "Subpicosecond Electrooptic Sampling: Principles and Applications", IEEE Journal of Quantum Electronics, Vol.QE-22, No. 1, January 1986, pp.69-78.
FIG. 1 is a diagram for explaining an example of a conventional voltage signal measuring apparatus using the electrooptic effect. In FIG. 1, a transparent substrate 5 is provided to apply a reference voltage to one main face of an electrooptic crystal 4, and a probe 7 is mounted on another face of the electrooptic crystal 4 via a reflecting electrode 6. The probe 7 makes contact with a wiring layer 17 which is provided on a substrate 16 such as a Si substrate, so as to measure a signal 18 which is applied to the wiring layer 17 and is to be measured.
A laser beam from a laser light source 1 is irradiated on the electrooptic crystal 4 via a phase plate 2 and a beam splitter 3. Reflected light from the reflecting electrode 6 is detected by photodetector 13 via a polarization beam splitter 11 and by a photodetector 14 via the polarization beam splitter 11 and a mirror 12. A differential output X is obtained from a differential amplifier 15 by inputting outputs of the photodetectors 13 and 14.
The differential output X is dependent on the signal 18 which is applied to the wiring layer 17, and the signal 18 can be described by V=a(X-b), where V denotes a voltage of the signal 18, a denotes a coefficient determining an amplitude of the signal 18, and b denotes a coefficient determining a 0 V position.
In this case, the refractive index of the electrooptic crystal 4 changes due to the Pockels effect depending on the signal 18 applied to the wiring layer 17. When the refractive index of the electrooptic crystal 4 changes, a polarization state of the laser light passing through the electrooptic crystal 4 changes, thereby causing a change in the ratio of a component reflected by the polarization beam splitter 11 and detected by the photodetector 13 with respect to a component transmitted through the polarization beam splitter 11 and detected by the photodetector 14.
In the above described voltage waveform measuring apparatus using the electrooptic crystal 4, it is necessary to obtain the coefficient b which determines the 0 V position, that is, the voltage origin, in order to measure the absolute level of the voltage waveform. In order to obtain this coefficient b, a correction pad having a known voltage is provided, and the probe 7 is made to contact the correction pad so as to obtain a correspondence between the voltage V of the signal 18 and the differential output X which is detected.
According to the conventional method of obtaining the coefficient b using the correction pad, the electrooptic effect (Pockels effect) is small. For this reason, the degree of modulation of the laser light in the polarization state is 0.1%, for example, and is extremely small. As a result, the voltage waveform measurement is greatly affected by a small change in the optical axis, a change in a characteristic of an optical element (such as, an electrooptic crystal) in response to a temperature change, and changes or disturbances in a signal processing system which processes the differential output X, and it becomes necessary to re-measure the coefficients a and b for accuracy.
However, if the probe 7 is controlled to contact the correction pad every time the re-measurement is made, it not only takes time to move the probe 7, but the optical axis shifts when the probe 7 is moved from the correction pad to a measuring point on the wiring layer 17, and it may become impossible to carry out an accurate measurement.
On the other hand, even if it were possible to accurately measure the coefficients a and b, it would still be necessary to accurately contact the probe 7 to the wiring layer 17 in order to carry out the voltage waveform measurement. However, Al or Al alloy is usually used for the wiring within the semiconductor device, and electrical contact between the probe 7 and the wiring layer 17 must be secured by breaking through a natural oxide layer formed on the surface of the wiring layer 17 by the probe 7.
In order to break through the natural oxide layer, it is necessary to apply on the probe 7 a load which is large compared to that at the time of the surface form measurement. But since the thickness of the wiring layer 17 becomes thinner as the width of the wiring layer 17 becomes narrower, it is difficult to control an optimum load on the probe 7 so as to achieve the electrical contact without damaging or breaking the wiring layer 17. Hence, even if it were possible to a skilled operator to control the optimum load on the probe 7, it would take a considerable amount of time to confirm the electrical contact between the wiring layer 17 and the probe 7.
In addition, even if the electrical contact between the wiring layer 17 and the probe 7 is confirmed, it is becoming more and more difficult to maintain satisfactory electrical contact for a predetermined time, due to displacements of the wiring layer 17 caused by vibration, thermal expansion or the like because the displacements cannot be neglected as the width of the wiring layer 17 becomes extremely small.